FIG. 1 illustrates an exemplary touch image captured by a touch sensitive device, such as a touch panel. In the example of FIG. 1, touch image 100 can include rows 101 and columns 102 of image data (depicted by small circles), where the image data values can indicate an area 110 of the device at which an object touched or hovered over the panel (depicted by a broken circle), and the remaining untouched area 120.
Two types of touch images that can be captured include an original touch image and a differential touch image, depending on the scan configuration of the device. FIG. 2 illustrates an exemplary column 102 from the touch image 100 of FIG. 1 showing what the image data 225 can look like in an original touch image and what the image data 235 can look like in a differential touch image. For explanatory purposes, the image data is shown in FIG. 2 as continuous, although it should be understood that the image data can also be discrete as illustrated in FIG. 1. In the original touch image, the image data indicative of an untouched area 225a can have a value of zero and the image data indicative of a touched area 225b can have values greater than zero, depending on the proximity of the touching or hovering object. The original touch image can include actual magnitudes of the image values. In the differential touch image, each column of image data can be balanced to have an average value of zero, i.e., having the DC information removed. As such, the image data indicative of an untouched area 235a can have a negative value and the image data indicative of a touched area 235b can have negative and positive values depending on the proximity of the touching or hovering object. As such, the column's negative and positive values can balance out to zero. The differential touch image can include differential (or relative) magnitudes of the image values.
A touch sensitive device capable of generating a differential touch image can have some advantages. For example, some device hardware used to generate an original touch image can be eliminated, thereby freeing up space to expand the device's viewing area. Also, image effects, such as thermal drift, can have little or no effect on a differential image. However, because many touch sensitive devices require the actual image value magnitudes (such as provided by an original touch image) to perform various functions, a differential touch image can have limited use.